Magnetization of Bi 2Sr 2CaCu 2O 8 single crystals : Field-induced diamagnetism above Tc, anisotropic irreversibility lines, and reversible mixed state properties

Abstract

We report on magnetization measurements made on Bi 2Sr 2CaCu 2O 8 single crystals with an 8T-SQUID magnetometer. The results confirm the existence of a broad field range with completely reversible characteristics for all temperatures above ∼ 15 K and for both field orientations. This is used to determine the penetration depths ( λ ab (0)=860 nm, λ c (0)=390 nm) as well as the thermodynamic critical field ( μ 0 H c(0)=231 mT). Detailed experiments near T c showed a large rounding of the transitions, which increased with increasing field and led to the observation of field-induced diamagnetism above T c. Attempts to evaluate H c2 in the sense of a mean field theory fail. The irreversibility lines are found to be anisotropic and occur at higher fields and temperatures for H∥ a, b.